A Beginner’s Guide to SWR
last updated 28 February 2025.
SWR is probably the subject of more misleading, incomplete, or simply incorrect statements than any other aspect of ham radio that I’ve encountered. Often this results from attempts to make it “simple to understand”, but it actually accomplishes the opposite. It doesn’t need to be that way.
My purpose for this beginner’s article on SWR is to get people started on the right foot, and to point out some of the false thinking that can lead the unwary astray. I’ll try to keep things simple, while still being accurate. There is more detail in the other related articles.
what is SWR?
SWR stands for Standing Wave Ratio. It is simply a measure of how well an antenna (or other load) matches the transmission line or transmitter design impedance. It doesn’t indicate anything about how well the antenna works (although a common symptom of a broken antenna may be a high SWR).
It’s easy to measure: many radios have built-in SWR meters, and there are many options for external meters (often offering forward and reflected power measurement as well), either as stand-alone units or built into antenna tuners.
A perfect match is an SWR of 1 : 1. The larger the first number, the higher the SWR.
It is always specified as a ratio of a number to one, like 3 : 1. Sometimes the last part is left off, and we might say “the SWR is 3”. But as a ratio, it is always some number to one, not the other way around. Yes, someone on the internet years ago decided that they preferred “1 : 3” for some reason, and we haven’t managed to eliminate that error. Yet. But it shows a lack of understanding of how the measurement is defined, and I wouldn’t want any readers of this web site to appear to be so ignorant.
why do we care?
When there is a good match (low SWR), power is (generally) transferred more efficiently from the transmitter to the antenna.
Many modern transmitters reduce output power when the SWR is too high. That helps to protect the transmitter against possible damage. So, in order to get rated power out of your transmitter, the SWR generally should be below about 2 : 1. In most cases, there is little additional benefit to getting it below 1.5 : 1, except that it looks nice on your meter. But you won’t notice it on the air.
Once your antenna is set up, a sudden change in SWR may indicate a problem with the antenna or the feedline. For example, the antenna may have fallen down, or the coax chewed by a squirrel. So SWR is one of the first things to check if your radio doesn’t seem to be working properly.
what if it is too high?
That depends…
If you are putting up an antenna for the first time, it may need to be adjusted for your installation. That’s what we call “tuning an antenna”. For example, a dipole antenna might need to be a bit shorter or longer than the formula length depending on the height above ground, the presence of other nearby objects, etc. Many commercial antennas have some form of adjustment to tune the antenna if the initial lengths aren’t quite right in your installation.
Tuning is a standard step when setting up a new antenna, and sometimes when moving an existing antenna. It’s often pretty simple to do. The first step is to measure the SWR at several points across the band. This is easy to do with some sort of Antenna Analyzer, but you can do it with your transmitter and an SWR meter, at least for the frequencies permitted by your license.
If the SWR is low enough across the whole band of interest, great! You’re done. If not, then you want to see if it is better at the high end or the low end. If your dipole has the lowest SWR at low frequencies, increasing as you go higher, then it is too long: try shortening it (folding back wire is more forgiving of mistakes than cutting it off) and repeat the measurements. If the SWR is better at the top of the band, then lengthen it. The object is to get the SWR low enough (often below 2 : 1) across the whole range.
Some types of antennas, particularly short ones, may not cover the whole band at a low SWR. For example, an 8′ mobile whip used on 80m may only cover 25 to 50 kHz of the band at low SWR. In that case, you want to adjust it to the frequency where you are most often going to operate.
what if it still isn’t low enough?
Yes, that can happen: the minimum SWR is at the right frequency, but the SWR is still a bit too high. Assuming you are adjusting a normal antenna and the SWR shifts with frequency as you would expect it to, this may be because of the environment. For example, a dipole mounted too close to the ground, or a VHF vertical beside a metal post. In that case, you can try changing the location or the way it is mounted to see if that makes a difference.
If the antenna is intended to present a 50 ohm load, and the SWR is way off, then inspect the antenna (and coax + connections) for a problem, possibly an assembly error, the wrong element lengths, a bad connection, or damage to the coax.
But some types of antennas are intended to be used with an antenna tuner: they aren’t expected to have a low SWR without one. This is more common with multi-band antennas, although some manufacturers may not always make it clear.
In that case. you can add an antenna tuner between the antenna and the radio, and adjust the tuner (or let it adjust itself, if it is the automatic type) to give you a lower SWR.
what else do I need to know about SWR?
- SWR is not an indicator of antenna efficiency. Using a long, lossy feedline will lower the SWR measured at the transmitter because more of your power is lost in the coax. And when coax cable gets water inside, the losses increase, and the SWR can actually improve due to the higher losses.
- SWR is not an indicator of antenna performance. There are a number of antenna designs where adjusting for lowest SWR can lower the gain in desired directions.
- High SWR, by itself, won’t damage most modern transmitters. But trying to operate for too long at high power into the high SWR might. I explain this in more detail in the main SWR article.
- Changing the length of your coax shouldn’t change the SWR (except for a small decrease with increased length due to the coax loss). If it does, then something else may be the problem.
- If you are trying to diagnose a problem with an antenna system, and are using an antenna tuner, check the SWR with the tuner out of the circuit.
- There are many tables available on the internet that purport to give the “additional loss due to SWR”. And many of them reflect a lack of understanding of transmission lines. If you find a table or calculator that doesn’t ask for the feedline type, length, and the operating frequency, don’t trust anything else you read on that web site. If it calculates based on the SWR, rather than the actual load impedance, then consider it just an estimate, although possibly a useful one.
BACK TO:
beginners’ guide to impedance
standing wave ratio (SWR) – main article
impedance – main article
RELATED ARTICLES:
the importance – or not – of antenna resonance
beginners’ guide to antenna tuners
how to adjust a manual antenna tuner
tuning an antenna with an antenna analyzer